Reversible tri-state structural transitions of hybrid copper(i) bromides toward tunable multiple emissions

Abstract

Hybrid copper halides have been rapidly developed because of their high photoluminescence quantum yield (PLQY), good water resistance, and variable structures; nevertheless, the controlled synthesis and interconversion of multiple hybrid copper halides containing different coordination geometries are rarely reported. In this study, using ethyltriphenylphosphonium as the templating cation [EtPh₃P]⁺, three stable hybrid copper bromides with diverse inorganic anion skeletons, (EtPh₃P)CuBr₂ (112), (EtPh₃P)₂Cu₂Br₄ (224), and (EtPh₃P)₂Cu₄Br₆ (246), were synthesized by carefully tuning the feeding ratio of CuBr and (EtPh₃P)Br. The emission peaks were located at 536, 546, and 580 nm with optical absorption edges of 3.64, 2.91, and 2.82 eV, respectively, showing a gradual redshift as the inorganic units change from a monomer (112) to dimer (224) and then to a tetramer (246). More interestingly, multiple solvents, including methanol (MeOH), ethanol (EtOH), isopropanol (IPA), acetic acid, ethylene glycol, and water with heat, can induce the spontaneous conversion of 112 and 224 to 246, which is attributed to the affinity of the organic salt with the solvent. Additionally, the crystal structure can transform from 246 into 112 (224) or can undergo a reversible dynamic conversion between 112 and 224 using the crystallization method. This work not only provides a strategy to adjust the inorganic units of the crystals but also to achieve multi-state structural transitions, making them viable candidates for next-generation multifunctional optoelectronic devices.